The invention pertains to a steering valve with a closed center for supplying a control element with hydraulic pressure, consisting of an input shaft, an output shaft, a torsionally elastic element that is connected at one end to the input shaft and at the other to the output shaft, a control pressure switching device and a torque/axial force converter unit.
Steering valves of this class are utilized in particular in power steering systems. The structure of such power steering systems is conventional. An input shaft connected to a steering linkage is connected to an output shaft equipped with a pinion. In a familiar manner, it is possible to insert a torsionally elastic element, a torsion rod, for instance, between the two shafts. The pinion acts on the toothed rack of a steering unit. The use of steering valves for such steering systems is familiar. For this purpose, a control pressure switching unit is provided, which may, for instance, be a valve casing engaged with the output shaft and surrounding the input shaft. Hydraulic fluid is pumped through the system with a pump. In case of a twisting of the input shaft relative the valve casing, hydraulic pressure is fed to a hydraulic motor, which assists the rack motion in one of the two possible directions.
Steering valves employed in power steering systems can be divided roughly into two groups. One type of steering valve employed in power steering systems is the steering valve with a so-called open center. In the neutral position of the steering valve with open center, hydraulic fluid at low pressure is pumped through the open valve into a reservoir by a pump. A twisting of the input shaft relative to the valve casing causes an elevation of the pressure, which is then fed to the servomotor. The disadvantage of steering valves with open centers is that a steady hydraulic flow must be maintained even in the neutral position. Thus, wasted power must be produced by the drive motor, derived primarily from the pressure accumulation and the volume flow.
Such a steering valve, which has all the features of this class, apart from the open center, is disclosed in DE 43 17 818 C1. A reaction piston with an electrohydraulic converter EHW is employed as a torque/axial force converter unit.
A power steering unit for passenger vehicles with a steering valve having an open center is known from DE 24 26 201 A1, wherein a hydraulic-bearing line is formed in the torsion element for purposes of minimizing the size of the unit, but not with regard to wear and tear on the seals.
Finally, DE 27 58 321 A1 discloses a steering valve with open center, in which an axial hole is drilled through the input shaft.
Another type of steering valve used in power steering systems is the steering valve with a so-called closed center. The hydraulic flow to the two cylinder sides is interrupted in the neutral position of the valve by the closed center position. Thus, there is no hydraulic flow in the neutral position. An operating pressure, which, upon opening of the valve only must be held within prescribed limits by the pump then going into action is applied to the practically closed valve by the pump. Because of the wasted power appearing for steering valves with open center even when the steering is not being operated, the steering valves with closed center are superior in the neutral position when considering power. They have the disadvantage, however, that practically no direct correlation exists between input torque and working pressure in the cylinder. In the prior art, therefore, additional measures were taken in order to provide the operator with a relationship between the steering force to be input and the resulting hydraulic pressure. Conventional seat valve arrangements are not without problems, since the seat valves undergo a certain wear and require considerable manufacturing and assembly effort. Moreover, they confront the operator with a threshold torque.
A steering valve of the generic class with a closed center is known from DE 28 34 421 A1. It has a torque/axial force converter unit and an axial borehole closed off by a seat valve for conducting hydraulic pressure. This additional seat valve not only increases the expense of manufacturing and assembly, it is also delicate and causes noise.
The special problem of steering valves with closed center with respect to the lack of a correlation between input torque and working pressure in the cylinder is expressed in steering initiation behavior, which is generally considered critical. Due to the fact that the hydraulic fluid is permanently present under maximum pressure at the control pressure switching device, a very high dynamism in the operation of the control pressure switching device results, since abrupt effects on system pressure occur.
Starting from this situation, the problem of the present invention is to improve a steering valve with closed center of the generic type such that its response dynamics are considerably improved, such that it can be made compact and reliable in its operation, and so that the steering valve can be simply and economically manufactured.